1. Field of the Invention
Methods and apparatuses consistent with the present invention relate to video interprediction encoding/decoding, and more particularly, to video encoding/decoding for rapidly performing interprediction on a current predictive picture or slice.
2. Description of the Related Art
In video compression techniques such as moving picture experts group (MPEG)-1, MPEG-2, or MPEG-4 H.264/MPEG-4 advanced video coding (AVC), video coding is performed using three types of pictures, i.e., an intra (I) picture, a predictive (P) picture, and a bi-predictive (B) picture.
The I picture is obtained by independent intra-coding within a picture without referencing to pictures that precede or follow the picture. For the I picture, time-axis motion estimation for removing temporal redundancy is not used and encoding is performed only using information obtained within the picture.
The P picture is obtained by forward interprediction encoding between pictures. In other words, encoding is performed by prediction between pictures using an I picture or a P picture as a prediction picture. At this time, prediction with respect to the current block included in the current picture is performed using only a temporally past picture.
The B picture is obtained by bidirectional interprediction encoding between a plurality of pictures. In other words, encoding is performed by prediction between pictures using temporally past and future pictures as prediction pictures. The B picture is different from the P picture in that encoding is performed using both a past picture and a future picture.
FIG. 1A illustrates interprediction with respect to a P picture according to the prior art.
Referring to FIG. 1A, for interprediction with respect to a current block 117 included in a current picture 113, a temporally past picture is used. For example, in FIG. 1A, a block 116 that is most similar to the current block 117 is searched for in a P picture 112 that immediately precedes the current picture 113 and the found block 116 is used as a prediction value of the current block 117. Only a residue, that is, a difference between the current block 117 and the prediction block 116, is encoded, thereby improving the compression rate of encoding.
Video encoding can be performed with the removal of temporal redundancy by consecutively generating P pictures for a single I picture. All pictures 111 through 115 between two I pictures are referred to as a group of pictures (GOP), and a single GOP is composed of 15 pictures according to the H.264 standard.
FIG. 1B illustrates interprediction with respect to a B picture according to the prior art.
Referring to FIG. 1B, a temporally past picture 122 and a temporally future picture 124 are used to predict a current block 127 included in a current picture 123 among pictures 121 through 125 included in a single GOP. The B picture is different from the P picture in that the current block 127 is predicted using blocks 126 and 128 included in a plurality of pictures.
As illustrated in FIGS. 1A and 1B, in order to predict the current block 117 or 127 using temporally adjacent pictures, the temporally adjacent pictures have to have already been decoded, as will now be described in detail with reference to FIG. 1C.
FIG. 1C illustrates interprediction with respect to a P picture according to the prior art.
Referring to FIG. 1C, in order to decode a current block 140 included in a current picture 135, remaining pictures 131 through 134 of a GOP, except for the current picture 135, have to be decoded. Since the current block 140 should be predicted by repetitive prediction using a block 136 included in the I picture 131, all pictures preceding the current picture 135 have to be decoded in order to decode the current block 140, causing inefficiency.
Such inefficiency does not become an issue if pictures are sequentially reconstructed and reproduced. This is because a next picture can be predicted by sequential decoding and reproduction. However, when the current picture 135 is reconstructed without reconstruction with respect to the previous pictures 131 through 134 illustrated in FIG. 1C, the inefficiency constitutes a problem. For example, when a user desires to start playback from a specific scene, pictures included in a GOP are not sequentially accessed and random access may be made to a particular picture of the GOP.
The inefficiency can be solved by predicting all the P pictures 132 through 135 included in the GOP directly from the I picture 131. This is because, for any P picture of the GOP, only the I picture is reconstructed and the current picture is predicted from the reconstructed I picture, and the current picture is then reconstructed.
However, such a solution may degrade the compression rate of encoding with respect to the current picture 135 because other blocks of the current picture 135 are not properly predicted from the I picture. For example, if a new image object that does not exist in the I picture appears in the picture 134 that immediately precedes the current picture 135 and in the current picture 135, the current picture 135 cannot use correlation with its immediately previous picture 134, resulting in degradation of compression rate.